T6 Condition Research Articles

Artificial age hardening behavior of a squeeze cast CoCrFeMnNi high entropy alloy reinforced 6082-aluminium matrix composite

An Mg-Si-based 6082 aluminium metal matrix composite reinforced with varying concentrations of CoCrFeMnNi high-entropy alloy particles (HEAp) was processed through stir-squeeze casting. The composites were heat treated to examine the age-hardening effect on microstructural changes, fracture mechanism, and an interface between HEAp and aluminium matrix. The evolution of microstructure and topography of fractured surface was investigated through field emission scanning electron microscope with energy dispersive X-ray spectroscopy and transmission electron microscope to characterize the precipitate and intermetallic compound formed at Al/HEAp interface. X-ray diffraction is used to characterize phase evolution. The tensile test was performed to investigate the correlation between tensile strength and artificial ageing. Under the T6 condition, composites exhibit superior mechanical properties such as improved hardness, yield strength (Ys), and ultimate tensile strength (UTS), maximum values were reported as 94.01 HRB, 303 MPa, and 486 MPa respectively for 8 wt% HEA/AA (T6) however, the ductility of composites is less as compared to the as-cast condition which is due to the formation of precipitates.

Electrochemical corrosion of stir cast and heat-treated magnesium alloy EV31A in different electrolytes: Analysis via X-ray photoelectron spectroscopy

The study aims to determine the chemical state of elements in EV31A after electrochemically corroded in four different electrolytic solutions. EV31A, a magnesium alloy containing rare earth elements, was manufactured via stir-casting and heat-treated based on T4 and T6 conditions. Electrochemical corrosion of the as-cast, T4, and T6 EV31A were carried out in 0.6 M NaCl, 0.05 M H2SO4, 0.1 M HNO3, and 0.1 M HF solutions. In addition, the X-ray photoelectron spectroscopy (XPS) examination was performed on the corroded surface of EV31A. During the XPS analysis, the elements such as Magnesium (Mg), Neodymium (Nd), Gadolinium (Gd), Zinc (Zn), Zirconium (Zr), chlorine (Cl), sulphur (S), nitrite (N), fluorine (F), and oxygen (O) were studied, and only the peaks of Mg, O, Nd, and Gd were detected on the surface of corroded alloy. In all test settings, the corrosion resistance of the as-cast alloy is better than that of the heat-treated alloy. Even though the corrosion resistance of the as-cast alloy is high, its corrosion rate of EV31A in H2SO4 and HNO3 is very high. The corrosion resistance of as-cast EV31A in NaCl and HF mainly results from the formation of Nd2O3 and electronegative fluorine, respectively, as determined by XPS.

Electron microscopic investigation of precipitation hardening in Al-Si based alloys

The present work was performed on Al-8.5%Si-1.75%Cu-0.55%Mg-0.2%Ti-0.32%Zr-0.015%Sr alloy (coded as 354 alloy) following T6 (at 160 °C) and T7 (at 240 °C) artificial aging conditions using high resolution transmission electron microscopy (HR-TEM). The main results inferred from the present study are the precipitation of GP zones in samples aged for 20 h at 160 °C along with S-Al2CuMg phase particles. Aging at 240 °C was carried out for different times, ranging between 30 min and 200 h. The quenched structures revealed a series of precipitations mainly GP zones at low aging times to Θ’-Al2Cu after 200 h together with the S-phase. No coherency was observed between the S-phase and the aluminum (Al) matrix. The growth of the S- phase takes place by precipitation of S2 type phase on the advancing S1-type phase. Mismatching was clearly observed at particle/matrix interfaces. High magnification images indicated the dissolution of some of the Θ” and Θ’ particles to enrich the nearby particles (coarsening). The start of incoherency is observed when the alloy is aged at 240 °C for 200 h.

Effect of Mn Addition on Microstructure, Mechanical Properties and Die Soldering of Rheocasting Al-7Si-0.3Mg Alloys

The effect of Mn additions (up to 0.8 wt.%) into Al-7Si-0.3Mg alloy on the microstructure, mechanical properties and die soldering was firstly investigated in gravity die casting conditions, with the aim to obtain an optimized Mn addition level. Especially, the stirring testing on H13 die material was used to elucidate the effect of Mn additions on the Fe concentration in the Al melting as a function of melting temperature (690 °C) and holding time (up to 32 h), which can provide an indirect sign of die soldering. On this basis, the addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy was found to be beneficial for microstructure, strength and die soldering. Secondly, rheocasting of Al-7Si-0.3Mg alloy without (Mn-free alloy) and with 0.6 wt.% Mn (Mn-containing alloy) was performed. The addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy was found to have following effects: (i) solidification temperature of α-Al is increased and a higher (about 20 °C) rheocasting temperature is therefore needed, but no significant die soldering problem was observed even with increasing casting temperature, (ii) Mn-containing phase forms along grain boundaries and within Al matrix, which increases tensile and yield strength but decreases elongation in as-cast, T5 and T6 conditions, (iii) number density and volume fraction of pores increases, and (iv) die soldering slightly improves. This paper demonstrates that the addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy is beneficial for microstructure, strength and die soldering in both gravity die casting and rheocasting conditions.

Fatigue behavior of notched and unnotched 7075-T6 aluminum alloy subjected to retrogression and re-aging (RRA) heat treatment and plasma nitriding

Aluminum alloys are widely used in aircraft components, which present the fatigue process as one of the main failure modes. The aeronautical industry has been searching for alternatives to increase the service life and safety of structural components. To address this issue, this work subjected the 7075-T6 alloy to retrogression and re-aging (RRA) heat treatment and plasma nitriding in order to obtain an improvement in the corrosion resistance. The objective was then to evaluate whether such treatments affected the fatigue behavior. Fatigue tests were carried out on the material with a smooth surface and with a notch (theoretical stress concentration factor of 5.7). A maximum likelihood estimation method was used to estimate the S-N curve parameters. The fatigue behavior without notch for the T6 and RRA treatments was similar, which is expected because the resulting microstructure of the RRA condition is composed of a homogeneous distribution of very fine η’ precipitates in the aluminum matrix, similar to the T6 condition. The worst fatigue behavior without notch was found for the material underplasma nitriding for two main reasons: the softening of the microstructure after processing and the substantial increase of the surface roughness. Regarding the notched material, the fatigue performance was reduced to a similar level regardless of the condition.

Adhesion properties of the hybrid system made of laser-structured aluminium EN AW 6082 and CFRP by co-bonding-pressing process

ABSTRACT A parameter investigation for manufacturing a hybrid system through the prepreg pressing process was carried out within the scope of this work to achieve optimal adhesion properties. The hybrid specimen comprises an aluminium sheet of alloy EN AW 6082 in T6 condition and a thermoset Carbon Fibre Reinforced Plastics prepreg. The prepreg pressing process allows the curing reaction of epoxy resin and the joining process to occur simultaneously to avoid an additional bonding process step. The surface of the aluminium sheet was pretreated in advance using a pulsed Nd:YAG laser to enhance the bonding properties. In the first step, the shear edge tests investigated the adhesion properties achieved with different consolidation (temperature, time and pressure) and laser parameters. Then, 3-point bending tests were carried out to investigate the influence of the consolidation parameters on the mechanical properties of the Carbon Fibre Reinforced Plastics-laminate. In this way, the optimal parameter sets for manufacturing hybrid structures were determined.

Effect of Stroke Length (50 mm and 100 mm) on the Dry Reciprocating Wear Characteristics of Refined and Unrefined A390 Alloy

The present work reports the influence of stroke lengths viz. 50 mm and 100 mm on the reciprocating wear characteristics of refined and unrefined hyper eutectic aluminium silicon alloy (A390). The refined A390 alloy was prepared through the electromagnetic stirring process along with the addition of 0.1 wt.% Al–5Ti–B grain refiner. The wear tests were performed in an indigenously developed pin on reciprocating plate tribometer conforming to ASTM G 133-05 standard. The microstructure had shown the refinement of eutectic silicon and decrease in size of polyhedral primary silicon particles due to the combined action of grain refiner addition and electromagnetic stirring. Refined and unrefined A390 alloys were heat treated under T6 condition. Hardness of the refined A390 alloy (107 BHN) was higher than that of unrefined A390 alloy (89 BHN). Reciprocating wear tests were conducted at two stroke lengths with contact loads varied from 15 N to 60 N and the linear velocity from 0.2 m/s to 1.0 m/s for a constant sliding distance of 350 m. During the wear tests it was observed that as the velocity varied from 0.2 m/s to 0.4 m/s the wear loss was found to decrease at both stroke lengths tested for refined and unrefined samples, further increase in velocity increases the wear rate. As the stroke length varied from 50 mm to 100 mm the wear loss was found to decrease for both refined and unrefined alloys. This reduction in wear loss with increase in stroke length was due to the reduced reversals of load happening at larger stroke length (100 mm) also the frequency of reciprocation is high at 50 mm stroke length compared to that at 100 mm for the similar sliding conditions tested.

High-temperature tribological performance of stir-cast and heat-treated EV31A magnesium alloy: Experiments and predictions

The temperature effect on the wear behaviour of EV31A Mg alloy during dry sliding wear was investigated. Wear tests were carried out at 50, 100, 150, 200, and 250 °C using a standard load of 10 N and a sliding distance of 1000 m. Weight loss method was used to calculate the wear rate. Optical microscopy was used to examine the microstructure of the EV31A alloy. FE-SEM with EDS analysis was used to investigate the wear morphology, and XRD analysis was performed both before and after the wear test. A high wear coefficient (K) value (more than 10−4) indicates extreme wear for EV31A in all the scenarios. T4 EV31A had a maximum wear rate of 20.2 mg at 150 °C. The as-cast EV31A alloy exhibits an excellent wear rate at the price of mechanical properties under all test scenarios. Wear resistance is improved by Nd and Zr oxides, although Mg and Gd oxides have little effect. Zn has no effect on the wear behaviour of the EV31A. In as-cast, T4, and T6 heat-treated conditions, the EV31A alloy exhibits delamination (abrasive wear), oxide development (corrosive wear), and delamination mixed with plastic deformation (adhesive wear). A Three-layered ANN and adapted Fine Gaussian SVM predicted tribological characteristics. In ANN prediction, the maximum R2 was 0.99 for CoF and 0.89 for wear rate, respectively. Despite the fact that the study's normal load is constant, machine learning models allow to deduce that temperature and normal load are the main influential parameters in CoF and wear rate, respectively.

Failure Investigation on Failed Towing Arm of an Aircraft: A Case of Improper Material Processing and Failed Repair Welding of a Cast Al-Cu-Si Type Aluminium Alloy

Present work describes the metallurgical failure investigation of a failed towing arm/bar. Aircraft attached with this incident travelled a distance of 2 km with normal towing speed (8 km/hr). The component is said to have broken while turning to right from taxi track and then turning left about 15-20o. The mating fracture pieces of the failed towing arm fork along with bent towing pin were analyzed for failure investigation. Parent material of the towing arm has been found to be a cast Al-Cu-Si alloy closely matching with the alloy 295 used in T6 condition. Pin is made of mild steel. Failure of the towing arm fork has been initiated by weld solidification cracking of the weld region at close to the fusion line. Liquation cracks at heat affected zone (HAZ) have aggravated the situation further by weakening the microstructure and facilitating the crack propagation. Grain boundary precipitates of Al-Cu-Fe type in HAZ have facilitated the propagation of failure along with the liquation cracking. On the other hand, Al-Fe-Mn type of precipitates sitting along the grain boundaries in weld area seems to have not facilitated the failure initiation unlike the solidification cracking. Improper material processing has led to the weakening of the bulk microstructure and thus the component itself, by introduction of various defects (pores, grain fall out, thick Al-Cu-Fe type of continuous grain-boundary precipitates and cracking). Weld repairing has been found to be a failure in Al-Cu-Si system in the present component.Keywords: Towing arm, cast Al-Cu-Si, repair welding, hot cracking, solidification cracking, liquation cracking, intergranular fracture, Al-Cu-Fe grain boundary precipitates, Al-Fe-Mn grain boundary precipitates.

Warm Aging of Pre-aged AA6013 Sheet and Its Relevance to Room Temperature and Warm Forming Applications—Experimental and Modeling Analyses

AbstractWarm forming has been shown to be an effective way to increase the formability of various aluminum sheet alloys. If applied to precipitation hardenable alloys, aging and/or coarsening may occur during heating and/or forming processes. As such, there is a potential to leverage the warm forming process to artificially age precipitation hardenable alloys. In this study, the aging characteristics of a pre-aged Al-Mg-Si-Cu alloy (designated AA6013-PA and comprising a 4 h, 100 °C pre-aging cycle following solutionization) were examined using: (i) room temperature tensile and (ii) bendability experiments, as well as (iii) elevated temperature tensile and formability experiments in the temperature range of 220-280 °C. In the pre-aged condition, the alloy exhibited very high work hardening, elongation and bendability. It was found that T6-like properties can be obtained with short duration secondary aging of the AA6013-PA starting temper (410 s at 235 °C), followed by a paint bake cycle (30 min at 177 °C). In such heavily aged conditions, the bendability is found to be moderate and consistent with a T6 temper. Bendability of the pre-aged alloy is shown to be enhanced by reducing the extent of aging, which may be beneficial for applications requiring energy absorption during crash, for example. In the pre-aged condition, AA6013 displayed a 55% improvement in room temperature formability, relative to the T6 condition, for near plane-strain loading using a Nakazima punch, although neither temper exhibited formability improvements through warm forming. The aging kinetics of AA6013-PA were determined from mechanical test data and used to calibrate an aging kinetics model. A strong obstacle model was used to relate precipitation state to yield strength, for artificial aging in the range 220-280 °C. The incremental strength increase due to exposure to an automotive paint cure cycle was also modeled and found to yield accurate results. Warm deformation, as well as room temperature deformation, were shown to enhance the paint bake response of the alloy, reducing the time to the peak aged condition by up to 14% for artificial aging at 177 °C.

Effect of high iron content on direct recycling of unhomogenized aluminum 6063 scrap by Shear Assisted Processing and Extrusion

Iron contamination in secondary aluminum scrap must be diluted with primary aluminum during conventional recycling to bring subsequent alloys within allowable Fe limits. In this study, Shear Assisted Processing and Extrusion (ShAPE) was investigated as a methodology for tolerating high Fe content during extrusion of secondary aluminum scrap. ShAPE was used to fabricate aluminum alloy 6063 tubing from secondary industrial scrap billets in the as-cast, unhomogenized condition. Iron content up to 0.3 wt% was investigated to explore the tolerance of ShAPE to ferrous contamination in the feedstock. Extensive refinement of Fe-rich second phases enabled tensile properties to meet, and in some cases exceed, industry standard values. For process temperatures of 470–530 °C, using unhomogenized billets with 0.3 wt% Fe and T6 properties yielded an average of 214 MPa yield strength, 243 MPa ultimate strength, and 15.5 % uniform elongation. For 0.2 wt% Fe and T6 condition, properties averaged 233 MPa yield strength, 260 MPa ultimate strength, and 16.5 % uniform elongation. These results suggest that ShAPE is a potential manufacturing route that can avoid the need for adding primary aluminum to dilute Fe during recycling.

Corrosion behavior of Al-6.8Zn-2.2Mg-Sc-Zr alloy with high resistance to intergranular corrosion

A high-strength and anti-intergranular corrosion (IGC) Al-6.8Zn-2.2 Mg-Sc-Zr alloy (AS alloy) was designed, and the effects of Sc addition and Cu removal on the microstructure, mechanical properties and corrosion behavior were systematically investigated. Based on transmission electron microscopy (TEM) and Kelvin probe force microscopy (KPFM) techniques, the underlying causes of IGC and exfoliation corrosion (EXCO) of the AS alloy were investigated, and the corrosion mechanism of AS alloy was finally obtained. The AS alloy exhibits excellent mechanical properties with a tensile strength of 601 MPa in the peak-ageing condition at 120 °C, which comes from the pinning effect of Al3(Sc, Zr) dispersions. Moreover, AS alloys of T6 and T74 conditions show outstanding IGC and EXCO resistance. On the one hand, the fine, near-ellipsoidal grains in AS alloy alleviate the peeling stress caused by IGC products by blocking the formation of wedge-shaped gaps along GBs, which then bring about improved EXCO resistance. On the other hand, the removal of Cu from the traditional Al-Zn-Mg-Cu alloy avoids the segregation of Cu into GBs and thereafter reduces the fluctuations of the potential and the electron work function (EWF) across the GBs; the addition of Sc induces the high fraction of LAGBs and thus weakens the solutes' segregation at GBs, which can be beneficial for smoothing the fluctuation of corrosion potential (Ecorr) to enhance the IGC resistance.

Development of a Novel High Strength Aluminum-Cerium Based Rotor Alloy for Electric Vehicle Induction Motor Applications

<div class="section abstract"><div class="htmlview paragraph">To increase vehicle range, light weighting of electric vehicles has been extensively researched and implemented by using aluminum intensive solutions. With regards to traction motors, aluminum alloys that have a desired combination of high electrical conductivity and strength are required for high power output and efficiency. In this research, a novel Al-Ce based alloy, with minor additions of Si and Mg for strengthening, was investigated in different heat treatment tempers to maximize mechanical properties while maintaining a high electrical conductivity. This new alloy system appears to have addressed the classic conundrum of the inverse relationship of mechanical performance verses electrical conductivity for traditional aluminum alloy systems. The results suggest that the Al-Ce-Si-Mg alloy had yield strength in excess of 120 MPa and electrical conductivity of at least 50 %IACS in the T5 and T6 conditions. Simulations of a squirrel cage induction machine show that this combination of properties resulted in similar motor performance to other rotor Al alloys. As a result, the Al-Ce-Si-Mg alloy is a promising alternative to the near pure Al or Cu-based alloys that are currently used in induction motor rotor castings.</div></div>

The impact of Ce-containing precipitates on the solidification behavior, microstructure, and mechanical properties of Al-6063

The present study investigates the effects of 1.0 wt% cerium (Ce) addition on the solidification behavior, microstructure, and mechanical properties of 6063 aluminum alloy. Thermal analysis exhibited four reaction points during the solidification of the reference 6063 alloy corresponding to the formation of α-Al, Al13Fe4, α-AlFeSi, and Mg2Si, respectively. Similar reactions were also observed for the Ce-modified alloy along with two additional reactions corresponding to the formation of Al-Ce and Al-Si-Ce precipitates. Microstructure examination revealed a grain size reduction of 25% due to Ce addition. In regards to the formed phases, the addition of Ce caused the formation of both Al-Ce and iron-free Al-Si-Ce precipitations. This suggested that the Si consumed during the formation of Al-Si-Ce precipitates was at the expense of Mg2Si rather than the silicon contained in Al-Fe-Si compounds. As for the mechanical properties, Vickers hardness measurements for samples in the T6 condition revealed an increase, in comparison to the as-cast condition, by 83% and 40% for the reference and Ce-modified alloys, respectively. The lower strengthening for the Ce-modified alloy was attributed to the less availability of Mg2Si due to silicon depletion. Compression test results for samples in the T6 condition exhibited a decline of both the yield and ultimate strength for the Ce-modified alloy, in comparison to the reference alloy. This was also attributed to the silicon depletion due to the formation of Al-Si-Ce precipitates, which impaired the precipitation hardening process.

Impact behavior of gravity cast AlSi10Mg alloy: Effect of hot isostatic pressing and innovative high pressure T6 heat treatment

In the present study the impact behavior of gravity casting AlSi10Mg alloy was evaluated with an instrumented Charpy pendulum. The effect of hot isostatic pressing, also followed by a T6 treatment, was analyzed in comparison with samples in the as-cast, annealed and T6 conditions. Furthermore, the effect of the innovative high-pressure T6 was investigated. It was found that the hot isostatic pressing is able to ensure densification of the alloy with an increase in both hardness and energy absorbed during impact. The T6 treatment performed at atmospheric pressure after the hot isostatic pressing is able to increase hardness and peak force. At the same time, the innovative high-pressure T6 is able to ensure similar results than those of hot isostatic pressing followed by T6, leading to a significant decrease in the treatment duration and costs and reducing the carbon footprint of the manufacturing process.

Microstructures and properties of wire-arc additively manufactured ultra-high strength aluminum alloy under different heat treatments

The microstructures, mechanical properties and corrosion behaviors of the ultra-high strength Al–Zn–Mg–Cu-Sc aluminum alloy fabricated by wire-arc additive manufacturing process using a self-prepared 7B55-Sc filler wire were systematically investigated under different heat treatments. The results showed that the microstructures of the as-deposited, T6, T73, and retrogression and re-aging (RRA) heat treatments were all composed of fine equiaxed grains with a size of about 6.0 μm. The grain boundary precipitates (GBPs) of the as-deposited sample were very coarse and continuously distributed along the grain boundaries, and the intragranular precipitates (IGPs) mainly consisted of a small amount of ηMg(Zn,Cu,Al)2 and η′ phases. After T6 heat treatment, the GBPs became much finer, but still continuously distributed along the grain boundaries. The IGPs mainly consisted of fine GP zones and η′ phases. After RRA heat treatment, the GBPs became coarser and discontinuously distributed along the grain boundaries. The IGPs were composed of η′ phases and ηMg(Zn,Cu,Al)2 phases. After T73 heat treatment, the GBPs became much coarser and sparsely distributed along the grain boundaries. The IGPs mainly consisted of coarser ηMg(Zn,Cu,Al)2 phases. The T6 heat-treated sample achieved the highest tensile strength of 618 MPa. While the strength of T73 heat-treated sample was sacrificed by up to 17% compared with the T6 heat-treated sample, but the corrosion resistance was the best of all heat-treated conditions. After RRA heat treatment, the strength was about 10% lower than the strength of the T6 condition, but the corrosion resistance was better than that of the T6 heat-treated sample.

Plasticity and ductility of an anisotropic recrystallized AA2198 Al-Cu-Li alloy in T3 and T8 conditions during proportional and non-proportional loading paths: simulations and experiments

The anisotropic material behaviour of a recrystallized AA2198 Al-Cu-Li alloy in T3 and T8 conditions was assessed by proportional loading of uniaxial-tension specimens in rolling (L), transverse (T) and diagonal (D) orientations. The width and longitudinal strains were measured to quantify plastic anisotropy. Notched-tension samples were tested in L and T directions. The material showed weak anisotropy in terms of stress strain curves and a moderate plastic anisotropy, consistent with its texture obtained by EBSD. An anisotropic Bron-Besson type material model was identified using this data base and a micro shear-only (SO) test. The model succeeded in predicting the behaviour of micro specimens for proportional tension-only (TO) loading and non-proportional load path changes including 'shear to tension' (ST) as well as 'tension to shear' (TS) tests. The non-proportional loading was achieved using a newly designed cross shaped sample. It was loaded in one direction, unloaded and subsequently loaded in the orthogonal direction till fracture. The average stretch to fracture of both alloys measured by a four point frame optical extensometer decreased by 29 % and 16 % for T3 and T8 respectively for the 'shear to tension' experiment compared to the proportional TO experiment. The average stretch to fracture of 'tension to shear' tests was reduced by 10 % for 2198T3 and hardly reduced for 2198T8 compared to the stretch to fracture of the SO tests, but subject to strong scatter. FE simulations showed local accumulated strain to fracture values that were similar for all loading histories for the T8 condition (0.73 − 0.84). Lower strain to fracture values were found in T3 condition (0.45 − 0.73), despite the enhanced macroscopic ductility in tension. This was attributed to larger less localized plastic zones, especially for the ST test. The ductility scatter was attributed to necking and damage development in tension that can affect strain localization, associated fracture path and ductility, as observed by DIC and fractography.

Influence of transition elements (Zr, V, and Mo) on microstructure and tensile properties of AlSi8Mg casting alloys

ABSTRACT The influence of the additions of transition elements (Zr, V, and Mo) on microstructure, precipitation behaviour and tensile properties of Al–8%Si–0.3%Mg permanent mould castings were investigated. It reveals that the presence of the transition elements increased the solute concentration in aluminium matrix, refined eutectic Si particles, and reduced the length and aspect ratio of intermetallic phases in as-cast microstructure. Moreover, the transition element addition promoted the formation of solute cluster and increased the number density of β′ precipitates and dispersoids during T6 heat treatment. The additions of Zr, V, and Mo considerably improved the tensile strengths and elongation in both as-cast and T6 conditions. The effects of the combined addition of Zr, V, and Mo on the microstructure, precipitation behaviour, and tensile properties were stronger than that of the addition of Zr and V.

Effects of Micro-Shot Peening on the Fatigue Strength of Anodized 7075-T6 Alloy.

Micro-shot peening under two Almen intensities was performed to increase the fatigue endurance limit of anodized AA 7075 alloy in T6 condition. Compressive residual stress (CRS) and a nano-grained structure were present in the outermost as-peened layer. Microcracks in the anodized layer obviously abbreviated the fatigue strength/life of the substrate. The endurance limit of the anodized AA 7075 was lowered to less than 200 MPa. By contrast, micro-shot peening increased the endurance limit of the anodized AA 7075 to above that of the substrate (about 300 MPa). Without anodization, the fatigue strength of the high peened (HP) specimen fluctuated; this was the result of high surface roughness of the specimen, as compared to that of the low peened (LP) one. Pickling before anodizing was found to erode the outermost peened layer, which caused a decrease in the positive effect of peening. After anodization, the HP sample had a greater fatigue strength/endurance limit than that of the LP one. The fracture appearance of an anodized fatigued sample showed an observable ring of brittle fracture. Fatigue cracks present in the brittle coating propagated directly into the substrate, significantly damaging the fatigue performance of the anodized sample. The CRS and the nano-grained structure beneath the anodized layer accounted for a noticeable increase in resistance to fatigue failure of the anodized micro-shot peened specimen.

Microstructure Evolution and Localized Corrosion Susceptibility of an Al-Zn-Mg-Cu-Zr 7xxx Alloy with Minor Cr Addition.

Microstructure optimization of Al-Zn-Mg-Cu-Zr aluminum alloys, particularly through recrystallization inhibition, for improved strength and corrosion resistance properties has been an important consideration in alloy development for aerospace applications. Addition of rare earth elements, sometimes combined with Cr, has been found to be beneficial in this regard. In this study, the role of a single addition of 0.1 wt.% Cr on microstructure evolution of an Al-Zn-Mg-Cu-Zr (7449) alloy during processing was systematically investigated by optical light microscopy, scanning electron microscopy, electron backscatter diffraction and scanning transmission electron microscopy. Susceptibility to localized corrosion after aging to T4, T6 and T76 conditions was evaluated by potentiodynamic polarization (PDP) measurements and an intergranular corrosion (IGC) test. A decrease in recrystallized fraction with 0.1 wt.% Cr addition was observed, which is attributed to the formation of Cu- and Zn-containing E (Al18Mg3Cr2) dispersoids and the larger as-cast grain size. Moreover, the depletion of alloying elements from solid solution due to the formation of the Cu- and Zn-containing E (Al18Mg3Cr2) dispersoids and η Mg(Zn,Cu,Al)2 phase at its interface affects grain-boundary precipitation. The observed decrease in localized corrosion susceptibility with minor Cr addition is correlated with the microstructure and equally discussed.